Octahedral molecular sieves of the type K-OMS-2 with different particle sizes and morphologies: Impact on the catalytic properties in the aerobic partial oxidation of benzyl alcohol

Abstract

Abstract Manganese-based octahedral molecular sieves of the type K-OMS-2 (cryptomelane structure) with different morphologies and specific surface areas in the range of 20–135 m2 g−1 were prepared via synproportionation of KMnO4 and Mn2+ salts, either in acidic aqueous suspension (reflux method) or by a solid-state reaction, and via oxidation of MnSO4 either by K2Cr2O7 or by molecular oxygen in aqueous solution. For the reflux method, the influence of K+ cations for the formation of the OMS-2 structure was proven. When KMnO4 is replaced by Ba(MnO4)2, the anion in the Mn2+ salt exerts a strong influence on the synthesis product. All materials were characterized by elemental analysis (ICP-OES), XRD, nitrogen sorption, scanning electron microscopy and TGA. For the oxidation of benzyl alcohol with molecular oxygen in liquid toluene at 110 °C, the catalytic activity of the K-OMS-2 materials is directly correlated to their specific surface area ABET. Since ABET increases with decreasing average crystallite diameter, the catalytic conversion presumably occurs at the outer crystallite surface. These relations are independent of morphology or synthesis procedure. The K-OMS-2 materials are more active than crystalline manganese oxides (MnO, Mn2O3, Mn3O4, β-MnO2), but less active than amorphous MnO2. Regeneration of deactivated K-OMS-2 catalysts can be achieved by calcining at 300 °C in air, partly due to the reversible desorption of water.